The present invention relates to an iron-base powder containing nickel and molybdenum, for producing sintered components with high demands on physical properties without the need of increasing the compacting pressure or sintering temperature above what is normal in powder-metallurgical manufacturing techniques.
Characteristic of the powder metallurgical manufacturing technique is the production in long series of components with good dimensional accuracy. The production chain starts by mixing a metallic powder, optionally with the addition of pulverulent alloying materials, with a lubricant to facilitate the subsequent pressing operation. In this operation, the powder is compacted at a pressure generally not exceeding 6 tons/cm.sup.2 to a blank whose shape closely or completely conforms to the shape of the finished component. The blank is heated and thereafter maintained at a temperature normally not exceeding 1150.degree. C. at which it is sintered and thus given its final properties, such as strength, toughness, density etc. Materials thus manufactured differ from materials manufactured by melt-metallurgical technique, substantially by their porosity, which may give relatively low density and have an adverse effect on the strength. In order to increase the strength of sintered components of iron powder without the addition of alloying materails, a substantial increase in compacting pressure and/or sintering temperature is necessary, which is costly and difficult to achieve since presses and pressing tools currently used are not intended for higher loads than 6 tons/cm.sup.2. Also, the maximum working temperature for the sintering furnaces is most often restricted to 1150.degree. C.
It is also known to use different alloying additives for increasing the strength of sintered steel.
At present, use is made substantially of two types of powder with alloying additives, namely powder mixtures and so-called atomized prealloyed powders. Powders mixtures are prepared by admixing in the iron powder a powder containing the alloying material, either in elementary form or as a compound which is decomposable during the sintering process. The atomized steel powders are manufactured by disintegrating a steel melt containing the desired alloying elements into powder. One of the drawbacks of powder mixtures is the risk of segregation which exists because powders having different characteristics, for example different particle size, are mixed with each other without being mechanically joined. This segregration entails a varying composition of the compacts manufactured from the powder mixture and, as a result, varying dimensional changes during the sintering thereof. Another drawback of powder mixtures is their proneness to produce dust, especially when the alloying material has a very small particle size, which may cause considerable pollution problems.
With the atomized powder, on the other hand, there is no risk of segregation whatever, since each powder particle has the desired composition of the alloy. Nor is the risk of dust formation very great since no elements of small particle size are included. The alloyed atomized powder however suffers from another major drawback, namely its low compressibility depending on the solution-hardening effect the alloying materials have on each powder particle.
A high compressibility is essential when it is desirable to obtain a component having high density, which is a prerequisite for high strength. The compressibility of a powder mixture, on the other hand, is practically the same as that of the iron powder included. This, together with the flexibility as regards the composition of the alloy that is characteristic of powder mixtures, has made powder mixtures the type of alloying powders most commonly used.
Today, the risk of segregation and dust formation can be almost entirely obviated by partial diffusion alloying or by using a binding agent for attaching the alloying elements to the iron particles, the graphite being also suitably bound in this manner without impairing the compressibility (SE patent application No. 8304832-2 (U.S. Ser. No. 732,045), SE Pat. Nos. 8001764-3 (GB Pat. No. 2,071,159) and 334,244).
The choice of alloying element is based on considerations which are well known within the powder-metallurgical field. One example is low contents of nickel and molybdenum and with an addition of copper in order to minimize the dimensional changes.
From SE patent application No. 7703382-7 (GB Pat. No. 1,510,455), it is previously known to produce a high-strength, sintered iron-molybdenum-nickel alloy with an addition of phosphorous. According to this SE application, sintering must however be performed at an elevated temperature (1250.degree. C.) to achieve an ultimate tensile strength of 600 N/mm.sup.2.